Lightweight structural panels are used in the construction industry, and are particularly desirable for outdoor use. The ideal panel is lightweight and relatively inexpensive to manufacture, has good insulation value, and is weather resistant. Panels that are durable, weather-resistant, and have high compressive and shear strength are particularly useful for outdoor applications. Structural panels having metal face sheets laminated with a plywood or wood composite core have been used, but are known to have the problem of being vulnerable to damage from moisture which can seep into the core. The moisture can cause the plywood or composite core to swell or rot. It is therefore preferable to replace the plywood or wood composite cores in structural panels for outdoor use.
Structural panels with metal face sheets bonded to a core of foam or a solid polyethylene sheet have also been used. Such a laminate can be formed with good adhesive bonding using an improved sheet bonding process developed by the present inventor and described in U.S. Pat. No. 5,114,510, entitled "Method of Forming a Lightweight Structural Panel", issued on May 19, 1992. While metal-faced panels with foam cores have been used successfully, they have insufficient shear strength for some high-load applications. Also, moisture collected in the foam core that is subjected to cycles of freezing and thawing can damage the panels by causing the layers to delaminate. On the other hand, use of a solid plastic core sheet is costly and wastes material in excess of what is necessary to provide adequate compressive and shear strength.
Honeycomb cores formed with contiguous hexagonal cells laminated between outer face sheets have also been used. However, the fabrication costs of honeycomb cores are relatively high. Therefore, honeycomb-core laminates, for example, as sold by Unida Corp. and Ciba Geigy Corp., are typically used in aircraft and other high-priced constructions, but are too costly for widespread use as construction material. The honeycombs cells are also aligned perpendicular to the face sheets, and thus offer very thin surface areas for bonding to the face sheets. This is a disadvantage since very careful gluing procedures and expensive high-strength adhesives are required. The cells are also closed off by the face sheets, thereby forming trapped air or moisture pockets which can generate expansion pressures from heat or freezing water that can delaminate the face sheets from the cores.
A structural panel formed of plastic face sheets fusion bonded to a corrugated plastic core sheet has also been used, for example, as described in U.S. Pat. Nos. 3,837,973 and 3,999,928 to Asakura et al. However, this type of corrugated all-plastic laminate does not have sufficient rigidity for many construction applications. Another type of structural panel has plastic face sheets bonded to a foam core which can retain its shape despite changes in temperature and humidity, for example, as described in U.S. Pat. No. 4,133,930 of the present inventor. While such plastic foam-core panels are resistant to weathering, they do not have sufficient shear strength for many construction applications.